Automobiles have become an indispensable element in modern human life, and the air pollution caused by their emissions has become increasingly serious, and directly threatens human health. Therefore, solving the problem of environmental pollution of motor vehicles has become a major task at present. Gasoline quality upgrade is one of the important methods to reduce motor vehicle emissions.
The difficulty in upgrading gasoline quality is not only to reduce the content of sulfur and olefin, but also to ensure the low loss of octane number and high yield of gasoline. Sulfur and olefins in gasoline are almost entirely derived from the catalytic gasoline, therefore, reducing the sulfur and olefin content of catalytic gasoline and ensuring low loss of octane number and high yield become the difficulties in upgrading gasoline quality. In the gasoline quality upgrade of the National VI Standard, reducing olefin while preserving octane number of gasoline become the focus of attention of human. The gasoline standards of National V Standard and National VI Standard are shown in Table 1 below. (Note: The National VI Standard is a proposed scheme at present, two schemes have been proposed, and the specific scheme has yet to be determined).
TABLE 1Comparison of gasoline standards of National V Standard and National VI StandardGasoline standardThe variation of NationalNational VNational VINational VIVI Standard comparing toStandardA StandardB StandardNational V StandardSulfur content, mg/kg, no101010greater thanBenzene content, % (V),10.80.8reducing by 0.2 percentageno greater thanAromatic hydrocarbon403535reducing by 5 percentagecontent, % (V), no greaterthanOlefin content, % (V), no241815reducing by 6-9 percentagegreater thanOxygen content, % (m),2.72.72.7no greater thanMethanol content, % (m),0.30.30.3no greater than
China is a big country in catalytic cracking, and catalytic gasoline accounts for 70% in the gasoline pool. In order to meet the requirement of upgrading quality and reducing olefins in the gasoline National VI Standard, the proportion of the compositions free of olefin such as reformate, aromatic oil, alkylated oil, isomerized oil and so on in the gasoline pool will increase in the next few years, and it is optimistically estimate that the proportion of the catalytic gasoline will be reduced to 60%. In addition, the olefin content in the catalytic gasoline can be reduced from 40% to about 30% by applying olefin-reduction technologies on a catalytic apparatus, for example, by applying technologies such as MT process, an olefin-reduction catalyst, LTAG and the like on the catalytic apparatus.
After adopting the above two means, it requires that the olefin content in the catalytic gasoline with an olefin content of 30% should be further reduced by 6-8 percentage (that is, the olefin content should be reduced to 22% or less) during refining process in order to meet the National VI factory standard of olefins. If the requirement for the olefin content is satisfied by simply increasing the depth of hydrogenation, the loss of octane number will be unbearable.
In the prior art, a Chinese patent ZL201310581366.8 applied by Hebei Refining Technologies Company Limited discloses a catalytic gasoline desulfurization technology for selectively preventing olefins from large loss of octane number. The technology cuts the catalytic gasoline into three fractions of a light, medium and heavy fraction. Among them, the olefin content in the light fraction is the highest, and the sulfur content of the fraction can be 10 ppm or less only by cutting after performing pre-hydrogenation treatment on the catalytic gasoline (<40° C., mainly C5). As for the middle fraction (40-100° C., mainly C6-C7), it mainly adopts a liquid-liquid extraction and separation means to obtain sulfur-poor oil mainly composed of alkenes and sulfur-rich oil mainly composed of aromatic hydrocarbons and sulfides, and to reduce the sulfur content in the sulfur-poor oil to 10 ppm or less. The sulfur-rich oil is mixed with the heavy fraction (>100° C.), and then selectively hydrotreated to achieve a sulfur content of 10 ppm or less. In the method, the middle fraction is subjected to liquid-liquid extraction and desulfurization. The middle fraction is fed from the middle of the extraction tower, and a desulfurization solvent is injected from the top of the extraction tower. After multi-stage countercurrent contact, due to the over-dissolution of the solvent and relatively larger dissolved amount of the olefin therein, the saturated C5 enters from the bottom of the extraction tower, and is in fully contact with the desulfurization solvent dissolved with the olefins in the lower part of the tower, so as to displace the olefins therein, reduce the dissolution of the olefins in the solvent, and minimize the olefin content in the sulfur-rich oil.
The technology described above achieves the goal of the desulfurization and preserving octane number upgraded in the gasoline National V Standard, and shows excellent application effects in practice. However, in order to meet the requirements of the new National VI Standard for further reducing olefins, it is necessary to further propose a catalytic gasoline desulfurization method which can ensure preserving the loss of the octane number of the catalytic gasoline at the level of the National V Standard and further reduce the olefin.